This invention relates to a fine pattern forming method for forming an arbitrary resist pattern for high precision fine processing on a semiconductor substrate by electron beam or focus ion beam direct writing lithography, using a multi-layer resist system.
In fabrication of IC, LSI, etc., conventionally, patterns were formed by photolithography using ultraviolet rays. Recently, as the pattern size of LSI elements becomes finer, and along with the manufacture of ASIC, the electron beam direct writing lithography has come to be employed. Since the electron beam resist used in pattern forming is generally poor in dry etch resistance, and also because the pattern resolution is impaired by the proximity effect due to reflection of the electron beam from the substrate, a multi-layer resist system is used in the electron beam lithography. Of the multi-layer resist system, the double-layer resist has a silicon containing resist, which is not mixed therewith, applied on an organic polymer film, and the tri-layer resist has an inorganic film, mainly SiO.sub.2 or organic polysiloxane film (SOG) formed between the organic polymer film of the double-layer resist and the resist, and they both function to suppress the proximity effect due to reflection of electrons. In these multi-layer resists, however, since the organic film is very thick, a new problem arises, which is the charge-up effect of electrons by insulation film. As electrons are accumulated on the resist, organic film and SiO.sub.2, which are insulation films, pattern deviation, butting error, and field deviation occur, making it impossible to write patterns accurately.
In the electron beam lithography, thus, there are problems in the dry etch resistance of the electron beam resist, proximity effect due to electrons and others, and attempts have been made to solve the problems by introducing the multi-layer resist. When the multi-layer resist system is employed, however, the charge-up effect of electrons is manifest, and it is difficult to draw patterns accurately. To solve this problem with the multi-layer resist, therefore, a metal or in particular a metal foil with high electric conductivity is used in the intermediate layer. For example, instead of SOG or SiO.sub.2, a metal foil of Si, W or Al is used, so that the patterns may be drawn accurately while preventing chargeup. However, since metals are used in the resist process, there is a problem of contamination. Besides, for sputter evaporation of metal, the process is complicated and difficult, and it leads to problems in metal etching after pattern formation, and metal peeling, and a problem of drawing in which alignment is difficult because of a decrease of secondary electrons reflected from the substrate because of the presence of the metal foil. In focus ion beam lithography, too, similar problems to those in the electron beam lithography are known.